Hi,
For ShapeNode is property EnableStyledText and an ability to set bold/subscript/superscript position.

For DiagramLink such ability absents.
In general decision exists: to group separate node with link and use styled text with this node (such way I found in mindfusion.diagramming.demo.diagram examples.chemical synthesis tree)
But... it's not always handy.

Is there any hope in the next releases to see styledtext for link ?

Best
Vlad